The invention relates to a high-voltage transformer arrangement for converting a first, relatively low direct voltage supplied by a direct-voltage source to a second, relatively high direct voltage. A high-voltage transformer arrangement of this type can be used, for example, for charging a capacitor with high capacitance over a short period of time.
Different designs are known for high-voltage transformer arrangements. The essential elements of a high-voltage transformer arrangement involve a current inverter connected to the direct current source and generally formed by an arrangement of power-electronic switches, a transformer with primary coil and secondary coil on a transformer core, a switch control for triggering the current inverter, an output rectifier that is connected to the secondary coil of the transformer and a filter connected to the output rectifier.
For nearly all applications of a high-voltage transformer arrangement of this type, the most compact transformer design possible is normally desired. The size of the transformer above all is critical for the structural volume of the high-voltage transformer arrangement. Essential elements for dimensioning a transformer designed for a high-voltage operation are the required insulation between the transformer components, namely the primary coil or coils, the transformer core and the secondary coil or coils.
When viewing a traditional transformer, for example, where a voltage of 40 kv develops against ground on the secondary side, an insulation designed for 40 kv must be provided between the location of the secondary coil where the 40 kV voltage appears and the adjacent location, which has ground or mass potential, e.g. the transformer coil that is connected to ground potential. As a result, narrow limits are set for a compact geometric dimensioning of the transformer.
It is the object of the invention to provide a high-voltage transformer arrangement for which the transformer design can be simplified considerably as compared to known high-voltage transformer arrangements.
This object is solved according to the invention for this high-voltage transformer arrangement in that the transformer core is provided with several interruption points on the magnetic path, in which insulation sections are located, and in that the transformer core sections formed by the interruptions, which are preferably controlled by potential, together with associated coils or coil sections of primary and secondary coils, form separate primary and secondary systems that are separated with respect to potential.
The creation of several xe2x80x9cinterruption pointsxe2x80x9d in the transformer core creates two insulated sections of the transformer core that are completely separate with respect to potential. The term xe2x80x9cseveralxe2x80x9d means that at least two interruptions are necessary in order to create two sections that are separate with respect to potential.
The separation of the transformer core into several sections is accompanied by a complete spatial separation of the transformer into a primary system and a secondary system. This spatial separation makes it easier to insulate the area between the primary coil and the corresponding core sections or the secondary coil and the corresponding core sections. Separating the transformer according to the invention into primary and secondary systems that are insulated against each other results in an increase in the stray inductance with simultaneous tapping of the main inductance. This effect, which may not be favorable per se, can be utilized for optimizing the high-voltage transformer arrangement operation with a corresponding design for the current inverter connected to the primary system or the output rectifier connected to the secondary system.
The high-voltage transformer arrangement according to the invention can be used in power supplies with a direct-current voltage intermediate circuit.
For one special embodiment of the invention, a continuous insulator is arranged between primary system and secondary system. Starting with this basic measure, the primary systems of a plurality of transformers can subsequently be combined, in particular inside a joint housing, which then contains the transformer core sections for the individual transformers and the associated primary coils. The secondary system can be treated in the same way. The primary systems and the secondary systems can be disposed in separate housings or in a joint housing. The use of a suitable arrangement or external wiring makes it possible to control the individual sections of the transformer core or cores with respect to potential, such that the lowest maximum potential differences result between the potentials of respectively one coil and the potential of the core section assigned to this coil. The insulator disposed between the core sections must then insulate the different potentials of the core sections against each other. For this, individual core sections of the transformers can be interconnected with specific switching components (of the current inverter(s) or the output rectifier(s)), such that the interfering emission from the high-voltage transformer arrangement is minimized.
It is particularly advantageous if the energy in the inventive high-voltage transformer arrangement of a special embodiment is transmitted without intermediate storage in the transformer and based on the flux converter principle from the primary system to the secondary system. A storage choke coil with thereto-connected output capacitor is preferably connected downstream of the output rectifier. A storage choke coil can also be provided in each output branch of the output rectifier to reduce the voltage stress for the single choke coil.
Comments relating to special, advantageous embodiments of the invention are provided in the following, wherein these embodiments can be used separately or in any combination considered useful or obvious to the person skilled in the art.
The inventive concept of separating the transformer core into several sections, electrically insulating these sections and the associated creation of a primary system and a secondary system make it possible to have a variety of high-voltage transformer arrangement designs. One or several primary coils or primary coil sections can be assigned in the known manner to a transformer. In the same way, one or several secondary coils or secondary coil sections can also exist. The high-voltage transformer arrangement can be provided with one or several individual transformers.
In principle, the primary coils of several transformers can be parallel connected, wherein these primary coils are then supplied by the same direct-current source via a joint current inverter. However, the primary coils of each transformer can also be triggered separately with a separate current inverter. In addition, the primary coils can be connected in series.
The secondary coils of several transformers in the secondary system can be connected in series, wherein each secondary coil is assigned a separate output rectifier that has a separate output filter or is connected to a joint one. Also possible is a series connection of the secondary coils of the high-voltage transformer arrangement, which is then connected to a joint output rectifier.
In the following, several circuit variants for connecting the primary system or systems of a special embodiment of the high-voltage transformer arrangement are explained.
The transformer or the interconnected transformers are advantageously connected on the primary side with one terminal to the center tap of a half bridge, consisting of power-electronic switches that do not block in return direction, and with the other terminal to a capacitor.
The transformer of an alternative embodiment is connected on the primary side with each terminal to a center terminal of a1 consisting of power-electronic switches, which do not block in return direction. The power-electronic switches in particular comprise transistors. These transistors either comprise a hybrid integrated or parasitic diode, or the diode is antiparallel connected as discrete component. It is known that power transistors of this type are encumbered with parasitic capacitances. The above-mentioned increased stray inductance of the transformer in the high-voltage generator2 according to the invention preferably can be attuned to the parasitic capacitances of the switching transistors, so as to result in a resonant operation for the switching operations of the current inverter. If necessary, the capacitance must be increased with a discrete capacitor. The switching losses can be reduced considerably with a resonant operation of this type and the electromagnetic compatibility of the circuit can be improved. By attuning the stray inductance of the transformer to the parasitic capacitances of the switching transformer, it is possible to trigger a complete recharge of parallel capacitances prior to the closing of the transistor, which must take over the current, thereby minimizing the switching losses.
1Note: This sentence is incomplete in the original. 
2Note: xe2x80x9cHochspannungswandler [high-voltage transformer]xe2x80x9d is used iln all other instances in place of xe2x80x9chochspannungsgenerator [high-voltage generator].xe2x80x9d
Preferably measures are taken in the secondary system of the transformer for the high-voltage transformer arrangement according to the invention to ensure the voltage sustaining capability of the output rectifier. For that purpose, several diodes are connected in series, if necessary, in each branch of the respective output rectifier. In particular, respectively one capacitance and one resistance are parallel connected for each individual diode of the output rectifier. As a result, the diodes are dynamically and statically balanced with respect to the voltage to be blocked. These capacitors, which are parallel-connected to the diodes in the output rectifier, are preferably attuned to the stray inductance of the transformer, so as to form a series resonance circuit with a characteristic frequency that is somewhat lower than the switching frequency of the high-voltage transformer arrangement predetermined by the switch control.
As mentioned in the above, the separation according to the invention of the transformer into a primary system and a secondary system, insulated against the primary system, results in an increase in the stray inductance, which can be used advantageously. At the same time, the main inductance of the transformer is reduced. If this reduction in the main inductance is high enough to prevent achieving the desired operational behavior of the high-voltage transformer arrangement, it is possible according to one preferred embodiment of the invention to parallel-connect or series-connect the primary coils of a transformer for a high-voltage transformer arrangement containing several primary coils, wherein these primary coils can include one or several secondary coils. This so-called nesting results in an increase in the main inductance of the transformer.
In accordance with the above measure, one special embodiment of the high-voltage transformer arrangement has several secondary coils that are connected parallel or in series, wherein one or several primary coils are also included. For this, the coils of the secondary system can also be conducted out either individually or with a joint tap, so that several secondary voltagesxe2x80x94even completely insulated secondary voltagesxe2x80x94are made available.
As a result of the inventive separation of the transformer in a high-voltage transformer arrangement with an insulator or individual insulator sections inserted between primary system and secondary system, it is possible to meet relevant regulations for insulation design with reduced technical expenditure.
Insulating systems that are known per se can be expanded by the insulator sections between primary system and secondary system in the high-voltage transformer arrangement according to the invention. In particular, the thickness, expansion and edge shape of the interruptions in the transformer core can be designed in dependence on the required voltage sustaining capability between primary-side and secondary-side core sections of the transformer and the transformer coils.
The insulator sections in particular can consist of several different materials. The material combinations are selected so as to ensure on the one hand the required insulating characteristics and, on the other hand, take over special mechanical functions.
Specifically, the idea is to design the insulator sections of the transformer so as to form components of a housing that encloses the primary system and/or the secondary system. Thus, the switching components can be sealed hermetically against each other if necessary. In that housing, the insulator separating the primary system from the secondary system can form the container wall. The primary-side and the secondary-side regions of the housing, which are separated by a wall, can be filled separately with gas, a liquid, a powder or the like to improve, for example, the cooling and/or insulation of the switching components in these regions of the housing.
It is particularly advantageous if the switch is controlled with a phase-locked loop. The first input of this loop is supplied with a current detector signal, derived from the chronological course of the primary transformer current, while the second input is supplied with the output signal from the phase-locked loop is supplied via a delay element. For this, the power-electronic switches are triggered with the output signal and the complementary output signal from the phase-locked loop.
The phase-locked loop consists in a manner known per se of a phase detector or phase comparator, a low-pass filter and a voltage-controlled oscillator (VCO) that is connected downstream of the low-pass filter. The current-detector signal is detected by a current sensor in the primary coil circuit for the transformer.
The phase-locked loop ensures a continuous adjustment of the trigger signal generated by the control circuit for triggering the power-electronic switch. As a result of the delay element between output and input of the phase-locked loop, the signals for triggering the power-electronic switch, which are tapped directly at the output of the voltage-controlled oscillator for the phase-locked loop, ensure that these switches are always switched just before the zero passage of the switch current. The current inverter thus is ensured to operate inductively and a switching can be realized without loss. To achieve a switching without loss, the delay is adjusted such that the energy stored in the stray inductance at the moment of interrupting the current inverter is high enough to completely recharge the parasitic capacitances of the power-electronic switch. Following this, the current runs freely over the antiparallel diode of the newly closed transistor and the voltage over this transistor is clamped to the diode flux voltage. As a result, the respective switch can be closed without losses. The switches are subsequently triggered with complementary signals, meaning signals offset by 180xc2x0, wherein short-circuits in the current inverter are avoided as a result of the lag time between the edges of the trigger-signals for the power-electronic switches, so that the desired complete recharging of the transistor capacitances is possible.
The adjustment of the upper and lower edge frequency of the voltage-controlled oscillator in the phase-locked loop preferably occurs in such a way that with a suitable lower edge frequency, the triangular current course present during the excitation phase does not exceed a maximum current value. The upper edge frequency is selected such that the clocking frequency of the current inverter is always higher than the characteristic frequency of the current inverter, which is predetermined by the stray inductance of the transformer and the capacitors that are parallel connected to the output rectifier.
As previously explained in the above, several transformers in the high-voltage transformer arrangement according to the invention can preferably be provided with separate current inverters. Modules can thus be formed, which respectively consist of a current inverter, a transformer primary system, a transformer secondary system, an output rectifier and an output filter, preferably comprising a smoothing choke and a capacitor.
Especially when using several modules of this type, the components for the secondary parts can be dimensioned for smaller voltages. The individual modules can respectively be operated and tested separately and individual modules can be replaced.
According to one special embodiment of the invention, which is particularly suitable for the modular transformer design, several (N) current inverters are cyclically clocked with the same frequency, but with offset phase angle relative to each other. In particular, a phase angle of 360xc2x0/N is provided between the individual reversing operations of the current inverters.
Several identical modules of this type are preferably operated in the stationary mode and with the same clocking frequency, wherein the phase position of the trigger signals for the current inverters of the individual modules is fixed on the basis of the operation of a single module. The primary transformer current course of a single module determines the operation of the remaining transformers (master-slave principle).
Through a cyclical clocking of several current inverters with signals having the same frequency, but offset phase angles, the rippling (ripple) of the current for the intermediate-circuit capacitor and thus also the thermal stress of the capacitors in the intermediate circuit can be reduced. The cyclical switching through of the individual current inverter operations can be realized with the aid of the above-mentioned master-slave principle by connecting a logic circuit to the output of a phase-locked loop, preferably via a frequency divider. By using a suitable logic circuit of the type of a ring counter, trigger signals that are staggered in time are generated for the individual (N) current inverters. With each current inverter, the output signals and the complementary output signals of the phase-locked loop are used as trigger signals, that is to say during half a period duration of a cycle (output signal) or a complementary half period of a cycle (complementary output signal). Attention must be paid to the fact that the signals belonging to a transformer or a current inverter do not overlap.
The above-mentioned frequency divider, in particular, operates with a division ratio TV=2*N (N represents the number of transformers or current inverters that are interconnected to form a module), wherein a decimal ring counter is connected downstream of the frequency divider. The decimal ring counter counts from zero to 2*Nxe2x88x921 and is provided with a separate output for each state.
For example, with N=5 transformers and current inverters, ten outputs of the decimal ring counter are connected in pairs to a total of five RS flip-flops, wherein each flip-flop delivers at its output or its complementary output a trigger signal or drive signal or a complementary trigger signal or drive signal for an associated current inverter.
To ensure that all transformer sections have the desired potential, cores made from core materials with poor electrical conductivity, e.g. ferrite, must be provided with a conducting layer that should not, however, encompass the magnetic flux.
This conductive layer usefully is designed so as to permit a good insulation relative to the associated coil (primary coil or secondary coil). The conducting layers functioning as potential control electrodes or, with core materials having high conductivity, the individual core sections are electrically coupled to each other via impedances and connected to the desired electrical reference potentials. This measure permits an easy adjustment of the desired potential ratios, meaning the lowest possible maximum potential difference is achieved between the potentials for one coil and the potential of the associated transformer core section.
The conductive layer on the transformer core sections can be used advantageously for adjusting the desired potential ratios without using additional elements for the potential control. To do so, several transformers of a high-voltage transformer arrangement are arranged in such a way that the desired potential ratios adjust themselves through the capacitive effect of the potential control electrodes.